It is known in the art that internal combustion engines nowadays used have a tendency to show high fuel efficiency when each cylinder operates under high load condition. Thus, for improvement in overall fuel consumption of the engine, a technique has been put into practice wherein under light load condition of the engine, some of the cylinders are stopped from being fed with fuel for applying higher operation loads to the other firing cylinders. One of the engines using the above technique is constructed such that when the engine runs on only selected cylinders under light load condition, the engine exhaust gas having generally the same pressure as the atmospheric air is introduced into the unused (or unfired) cylinders to minimize the pumping loss caused by the pumping movement of the unused cylinders. This engine shows a better result in fuel consumption.
However, these engines mentioned above cause inevitably increase of NO.sub.x contained in the exhaust gas because of application of higher operation loads to the firing cylinders.
It should be noted that, hereinafter, the term "all cylinders working condition" refers to a condition wherein all cylinders of the engine are fired to generate output power, while the term "selected cylinders working condition" refers to a condition wherein only selected cylinders are fired to generate the output power.